I do think the fanfic is not a bad idea - and I'm a person who would find "give yourself permission to write really badly" helpful, because I'm a perfectionist. But, if that doesn't work for you, maybe you could just try writing something else? Some form or subject you've never attempted before? Haikus? Maybe the opening sentences of twenty different novels? And then choose five, and try for a paragraph?

It seems the point is to get back to being playful, which is hard to do when you've been doing such focused and concentrated work for so long. But you'll get there! Something will break the logjam. Keep trying different things - journals, wordgames, bad poetry, whatever - something will work. I'm sure of it! Because you are a born writer, aren't you?